Improved file handling especially property file

[volkm]

• Consistently use openFile / closeFile helper functions
• Introduce namespace storm::io for all files in storm/io
• Warn if a property string looks like a link to a property file but the file does not exist (Resolves Confusing error message for --prop option #643). The check warns if the property strings contains any of .prop, .prctl, .pctl or .csl (following the Prism documentation)

[volkm]

Each call to parseProperties currently checks whether a potential file with this name exists. File I/O operations can be costly and this can introduce some unnecessary overhead. I would suggest to add dedicated functions for parsePropertiesFromString and parsePropertiesFromFile to avoid this overhead if the input format is clear.